This invention relates generally to spark-ignited engines. More particularly, this invention relates to a device for exhaust gas purification for spark-ignited engines.
Exhaust emissions from spark-ignited engines, especially from small spark-ignited engines such as those used in lawn mowers, small generators, floor cleaning machines, are significant sources of harmful pollution gases. In contrast to automotive engines which are equipped with a closed-loop air/fuel ratio control system, such engines operate without an air/fuel ratio control system in a fuel-rich environment, and are much dirtier than automotive engines. In order to reduce harmful exhaust emissions, a three-way catalytic (TWC) converter is widely used. With fuel-rich combustion, the TWC converter cannot reduce carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxides (NOx) simultaneously at its maximum efficiencies due to a lack of the oxygen which is needed for full catalyst functionality. Hence, it is desirable to introduce a certain amount of secondary air into the exhaust stream ahead of the TWC catalyst/converter to compensate for the oxygen shortage in the engine-out emissions
There have been some exhaust emission purification devices using Venturi devices to help introduce secondary air into raw exhaust and enhance TWC catalyst efficiency. These Venturis use a traditional type of structure including a tubing or narrow channel with a convergent contraction structure (a throat) in the middle, and a small passage or multi-holes/apertures connecting the Venturi throat to the ambient in order to draw in secondary air. The exhaust flow stream goes into and exits the Venturi in line. The secondary air is introduced usually perpendicularly to the main flow stream of the exhaust. Due to exhaust pressure pulsation, there is often a back-flow or blow-out problem, i.e. the untreated harmful exhaust gases are blown out through the secondary air passage prior to entering the TWC catalyst whenever the exhaust pressure is higher than the ambient pressure. Another challenge is to compromise pressure drop and vacuum level for best performance.
In general, therefore, the performance of prior art devices has been lacking, and there is a need for an alternative device to provide suitable exhaust gas purification for spark-ignited engines.